In recent years the need for and therefore the relevance of wireless communication has increased steadily. Nowadays wireless communication is an inherent part of all kinds of businesses as well as of the day to day communication. Accordingly, as the number of users connected has been multiplied over the past, the amount of data transmitted over wireless communication systems has also been multiplied, resulting in demands for higher capacity, higher data rate and multimedia services. Therefore, optimization of bandwidth usage within networks became an issue.
For example, the IEEE standard 802.16-2004 (IEEE Standard for Local and metropolitan area networks—Part 16: Air Interface for Fixed Broadband Wireless Access Systems), so called WiMAX standard, has been designed to satisfy various demands for higher capacity, higher data rate, and advanced multimedia services.
In order to support multimedia services with variable requirements of quality of service (QoS) a scheduling algorithm has to be provided. Particularly, an efficient uplink (UL) scheduling algorithm for voice services is required because voice services are delay sensitive and have an important portion within the multimedia services. As a technology for voice services in a packet oriented architecture, for example voice over IP (VoIP) technology can be utilized.
To support the variable requirements of QoS, various scheduling algorithms have been proposed, e.g. unsolicited grant service (UGS), a polling service for real-time, e.g. real-time polling service (rtPS), an extended polling service for real-time, e.g. extended real-time polling service (ertPS), a polling service for non-real-time, e.g. non-real-time polling service (nrtPS), best effort service (BE), etc. The unsolicited grant service, real-time polling service and extended real-time polling service algorithms are designed to support real-time services, while the non-real-time polling service and best effort service algorithms are designed to support non-real-time services.
A communication network comprises at least one base station (BS) and at least one subscriber station (SS). The BS is a generalized equipment set providing connectivity, management, and control of the subscriber station. The SS is a generalized equipment set providing connectivity between subscriber equipment and a base station.
In the IEEE standard 802.16 a contention resolution for the bandwidth request (BR) transmission in uplink channel is provided. In the sense of the application, uplink is the direction from a subscriber station to a base station. As contention resolution for a bandwidth request (BR) the following approach can be exemplarily selected. The BS controls assignments of bandwidth on the uplink channel through messages, e.g. UL-MAP messages, and determines which slots are subject to contentions. An UL-MAP is a set of information that defines the entire access for a scheduling interval. A method of contention resolution as proposed in the WiMAX standard is based on a truncated binary exponential backoff based on contention windows (CW), with the initial backoff window and the maximum backoff window controlled by the BS. The values of the CWs are specified as part of the UCD message and represent a power of 2 (2^n) value. For example, a value of 4 indicates a contention window between 0 and 15 (2^4); a value of 10 indicates a contention window between 0 and 1023 (2^10).
When a SS has information to send and wants to enter the contention resolution process, it sets its initial backoff window equal to the parameter request backoff start defined in a message for an uplink channel descriptor (UCD), referenced by the UCD Count in the UL-MAP message currently in effect. The UCD is a medium access control message that describes the physical layer (PHY) characteristics of an uplink. The parameter request backoff start is the initial backoff window size for contention bandwidth (BW) requests, expressed as a power of 2 (2^n) value.
Then the SS shall randomly select a number within its backoff window. This random number indicates the number of contention transmission opportunities that the SS shall defer before transmitting. After transmission of the information, the SS waits for an information element (IE) of the data grand burst type, e.g. data grant burst type IE in the subsequent UL-MAP message. The data grant burst type IEs provide an opportunity for an SS to transmit one of more uplink protocol data units (PDU). A PDU is a data unit exchanged between peer entities of the same protocol layer. These IEs are issued either in response to a request from a base/subscriber station, or because of an administrative policy, such as unicast polling, providing some amount of bandwidth to a particular base/subscriber station. Once the data grant burst type IE has been received, the contention resolution is complete.
The SS shall consider the contention transmission lost if no data grant burst type IE for this SS has been received in the subsequent UL-MAP messages. The number of the subsequent UL-MAP messages the SS analyzes is specified by the parameter Contention-based reservation timeout. The SS shall then increase its backoff window by a factor of two, as long as it is less than the maximum backoff window. The SS shall randomly select a number within its new backoff window and repeat the deferring process described above.
Among the above mentioned scheduling types, the unsolicited grant service and the real-time polling service are not allowed to use a method for contention resolution for bandwidth requests to reserve uplink resources. The unsolicited grant service is designed to support real-time data streams consisting of fixed-size data packets issued at periodic intervals, such as voice over IP services without silence suppression. The real-time polling service is designed to support real-time data streams consisting of variable-sized data packets that are issued at periodic intervals, such as video.
On the other hand, for the non-real-time polling service and best effort service, the Request/Transmission Policy setting shall be set such that the SS instead of the BS is allowed to use contention request opportunities. The non-real-time polling service is designed to support delay-tolerant data streams consisting of variable-sized data packets for which a minimum data rate is required, such as file transfer protocol (FTP). The best effort service is designed to support data streams for which no minimum service level is required and therefore may be handled on a space-available basis.
The extended real-time polling service has been designed to support real-time service flows that generate variable size data packets on a periodic basis, such as for example voice over IP services with silence suppression. The scheduling mechanism is built on the efficiency of both unsolicited grant service and real-time polling service. The BS shall provide unicast grants in an unsolicited manner like in unsolicited grant service, thus saving the latency of a bandwidth request. However, whereas the data packets of unsolicited grant service are fixed in size, the data packets of extended real-time polling service are variable in size.
In extended real-time polling service, the SS informs the BS of its voice status information using a grant management subheader in case that its data rate of the voice codec is decreased. According to the voice status, the BS can reduce the polling size. However, in case that a data rate of the voice codec is increased, the SS can not send the respective voice data packet by using assigned polling resources. The SS has first to request additional bandwidth prior sending the voice data packets using bandwidth request header. The BS may not change its polling size without any prior indication or request from the SS.
In conclusion, when collision of data packets or bandwidth requests occurs, the respective bandwidth request in contention may belong to extended real-time polling service, non-real-time polling service, best effort service, and also some medium access control layer (MAC) signalling messages such as handover. Unlike the non-real-time polling service and best effort service, the collision of bandwidth request for extended real-time polling service influences the speed of data rate, therefore decreasing the time-critical data flow, so that the QoS can not be guaranteed.
Accordingly, in the above mentioned scheduling types the problem remains that all bandwidth requests, irrespective of the utilized scheduling algorithm/service type (extended real-time polling service, non-real-time polling service, best effort service), are treated equally and therefore any QoS requirements for time-critical services can not be met.